Proposals for future federal, state and local regulations on controlling emissions from internal combustion engines generally call for increased reductions in nitrogen oxide (NO.sub.x) while keeping particulate emissions at or below current levels. Representatives of the diesel engine industry and some regulatory agencies have signed a Statement of Principles document which calls for combined NO.sub.x and hydrocarbon (HC) emissions of 2.5 grams per brake horse power-hour (g/bhp-hr) or less and particulate matter emissions of 0.10 g/bhp-hr or less by the year 2004. The U.S. Environmental Protection Agency (EPA) issued a notice of proposed rule making entitled Control of Emissions of Air Pollution from Highway Heavy-duty Engines (61 F.R. 33421, Jun. 27, 1996) with proposed changes to 40 C.F.R. Part 86 based in part on this Statement of Principles.
In the past significant progress has been achieved in reducing diesel engine emissions by various changes in engine design and fuel system design. Fuel improvements and exhaust after treatment techniques have also been used to meet the challenge of lower allowable levels of engine exhaust emissions. At the same time, customers are demanding greater fuel efficiency and extended engine life with fewer maintenance requirements. As a result, several difficult design tradeoffs must often be made to meet these sometimes conflicting goals. For example reducing NO.sub.x emissions from a diesel engine by retarding injection timing may have a negative impact upon fuel economy. Also, design changes made to reduce particulate emissions may increase NO.sub.x emissions and vice versa. The task of maintaining good fuel economy is especially difficult with the need to control NO.sub.x and particulate emissions at the new, proposed relatively low levels in comparison with prior acceptable standards. A paper entitled Progress in Diesel Engine Emissions Control by Magdi K. Khair was presented at the ASME Energy-Sources Technology Conference and Exhibition during January 1992 in Houston and provided a summary of previous changes made to improve performance while reducing emissions from diesel engines.
One technology which shows promise in helping engine designers meet the objectives of reduced emissions with the same or improved fuel efficiency is exhaust gas recirculation (EGR). EGR technology has been used for some time in light duty diesel engines to effectively reduce NO.sub.x emissions to levels approaching those proposed as future standards. Exhaust gas recirculation reduces NO.sub.x in diesel engines by diluting the oxygen induced with the fresh charge air as well as acting as a heat sink in the combustion process. One side effect of increased EGR is often an increase in insoluble particulate matter, primarily soot, in exhaust gas exiting from the cylinders or combustion chambers. This increase in particulate matter often results in the need to add complex, expensive exhaust gas after treatment systems such as particulate traps to maintain low levels of particulate emissions from the associated diesel engine.
Conventional EGR systems for diesel engines generally employ a single control valve or metering valve which is typically located a considerable distance from the associated intake manifold. Conventional EGR systems often include a cooler disposed between the control valve and the associated intake manifold. As a result of this configuration, pipes, ducts or other types of fluid flow conduits with considerable volume are frequently installed between the single control valve of conventional EGR systems and the associated intake manifold. This volume must be filled or emptied by the conventional EGR system in response to changes in engine operating conditions. As a result of this volume, conventional EGR systems generally have a slow response time to changes in engine operating conditions. During such changes in engine operating conditions, exhaust gas may be supplied to the intake manifold when it is not required which may result in an unnecessary increase in particulate emission. Alternatively, exhaust gas may not be supplied fast enough to the intake manifold when it is required which may result in an increase in NO.sub.x emissions above desired levels.
Another problem associated with conventional EGR systems includes uneven distribution of exhaust gas between the individual cylinders of the associated internal combustion engine. A conventional EGR system having only a single control valve typically relies upon mixing within the intake manifold of the fresh air charge and the exhaust gas supplied by the EGR systems. Intake manifolds are generally not designed to optimize such mixing of air and exhaust gas, particularly when the ratio of exhaust gas to fresh air is relatively large. As a result, different amounts of exhaust gas may be supplied to respective cylinders of the associated engine. Uneven distribution of exhaust gas by conventional EGR systems tends to increase insoluble particulate emissions from the cylinders receiving larger portions of such exhaust gas.
For some applications, the amount of residual exhaust gas remaining in the cylinders or combustion chambers of an internal combustion engine may be varied in an attempt to control NO.sub.x emissions. Typically, the amount of residual exhaust gas remaining in a combustion chamber or cylinder will depend upon the timing for opening and closing of intake ports and exhaust ports associated with the respective combustion chamber or cylinder. However, such residual exhaust gas cannot be effectively cooled. Therefore, over all, engine operating efficiency generally declines as residual exhaust gas in a combustion chamber or cylinder increases.